1. Field of the Invention
The present invention relates to an automatic level control circuit for controlling input levels of a stereotape recorder etc. (hereinafter referring to as ALC circuit). More particularly, it relates to ALC circuit having a small output voltage unbalance between channels.
2. Description of the Prior Arts
FIG. 1 shows a conventional ALC circuit wherein as suffixes of symbols, a means the side of channel 1; b means the side of channel 2 and no suffix means a common circuit for both channels. In FIG. 1, the reference (1) designates a smoothing capacitor; (2) designates a resistor connected in parallel to the smoothing capacitor; (3) designates a base control resistor of an NPN transistor (4); (5) designates an emitter resistor of the transistor (4); (6) designates a vertical PNP transistor; (7), (8a) and (8b) respectively lateral PNP transistors; (9), (10a) and (10b) respectively designate emitter resistors of the transistors (7), (8a) and (8b), which are connected to the transistors (6), (7), (8a), (8b) to form a current mirror circuit. The reference (11a) and (11b) respectively designate each collector current control resistor of the NPN transistors (12a) and (12b); the references (13a) and (13b) respectively designate ALC transistors driven by the transistors (12a) and (12b); the references (14a) and (14b ) respectively designate input resistors; (15a) and (15b) respectively designate input coupling condensers; (16a) and (16b) respectively amplifiers; (17a) and (17b) respectively designate input terminals of the amplifiers (16a) and (16b); and (18a) and (18b) respectively designate output terminals.
The conventional ALC circuit has the above-mentioned structure. When an input signal vi is applied to the external input terminal (20a) and an external input terminal (20b), each output viG is given at each of the output terminals (18a), (18b). The outputs are converted into DC outputs by rectifying circuits (21a), (21b) and the DC outputs are applied through the resistor (3) to the base of the transistor (4), whereby the transistor (4) is in ON state. The current is fed through the collector of the transistor (7) into the transistor (4). The transistors (12a), (12b) are driven by feeding respectively equal current through the collectors of the transistors (8a), (8b) to the bases of the transistors (12a), (12b) by a current mirror circuit made of the transistors (6), (7), (8a), (8b) and the resistors (9), (10a), (10b). Each base current is fed to the ALC transistors (13a), (13b) by the transistors (12a), (12b) whereby the ALC transistors (13 a), (13b) are saturated to attain the ALC operation. The input levels at the input terminals (17a), (17b) of the amplifiers (16a), (16b) are attenuated to vi.multidot.Rs/(Ri+Rs) and the outputs of the amplifiers (16a), (16b) are respectively given as EQU vi.multidot.G.multidot.Rs/(Ri+Rs)
wherein Rs designates a saturated resistance of the ALC transistors (13a), (13b).
When the external input vi increases, the outputs of the amplifiers (16a), (16b) increase and a DC potential applied to the base of the transistor (4) increases whereby the collector current of the transistor (7) of the current mirror circuit and the emitter currents of the transistors (7), (8a), (8b) increase. When the emitter currents of the transistors (8a), (8b) increase, the emitter currents of the transistors (12a), (12b) increase and the base currents fed to the ALC transistors (13a), (13b) increase whereby the saturated resistances Rs of the ALC transistors (13a), (13b) decrease. That is, the ratio for attenuation Rs/(Ri+Rs) decreases to counterbalance the signal vi.multidot.Rs/(Ri+Rs) applied to the input terminals (17a), (17b) of the amplifiers to be a constant and the output of the amplifiers vi.multidot.G.multidot.Rs/(Ri+Rs) becomes a constant.
On the contrary, when the external input vi decreases, the output of the amplifiers becomes a constant under the same consideration.
Thus, the conventional circuits often cause unbalances between the output voltage of the channel 1 and the output voltage of the channel 2 in view of the difference of characteristics of V.sub.BE of the transistors (8a), (8b) or the difference of h.sub.FE of the transistors (12a), (12b). FIG. 2 shows an operation diagram for illustrating disadvantages of the conventional ALC circuit by numerical equations. For example, the currents I.sub.Ba, I.sub.Bb fed into the bases of the ALC transistors (13a), (13b) are given by the equations: EQU I.sub.Ba =I.sub.1a (1+h.sub.FEa) (1) EQU I.sub.Bb =I.sub.1b (1+h.sub.FEb) (2)
wherein the references I.sub.1a, I.sub.1b respectively designate emitter currents of the transistors (8a), (8b) and h.sub.FEa and h.sub.FEb respectively designate h.sub.FE of the transistors (12a), (12b).
The values I.sub.1b and h.sub.FEb are given by the equations: EQU I.sub.1b =I.sub.1a +.DELTA.I (3) EQU h.sub.FEb =h.sub.FEa +.DELTA.h.sub.FE ( 4)
wherein I.sub.1b &lt;I.sub.1a and h.sub.FEb &lt;h.sub.FEa.
The ratio of I.sub.Bb to I.sub.Ba is given by the equation: ##EQU1##
The saturated resistances Rsa, Rsb of the transistors (13a), (13b) respectively given by the equations: EQU Rsa=ka/I.sub.Ba ( 6) EQU Rsb=kb/I.sub.Bb ( 7)
wherein the references ka, kb respectively designate constants given by a size and a shape and V.sub.CE of each transistor.
The ratio of Vob to Voa as the outputs of the amplifiers (16a), (16b) is given as follows by the equations (5), (6) and (7). ##EQU2##
As a practical example, in the case of I.sub.1a =40 .mu.A, I.sub.1b =44 .mu.A, h.sub.FEa =100 and h.sub.FEb =110, it provides .DELTA.I=4 .mu.A and .DELTA.h.sub.FE =10 and EQU Vob/Voa=0.9.times.0.9=0.81 (9)
in view of the equation (8).
The output of Vob is only 81% to Voa.
As it is found by the equation (8), in the conventional circuit, the output voltage is affected by both of the difference of emitter currents caused by the difference of V.sub.BE of the transistors (8a), (8b) and the difference of h.sub.FE of the transistors (12a), (12b) whereby the unbalance of output voltages between channels disadvantageously increases.